The present application relates generally to an improved data processing apparatus and method and more specifically to hybrid superconductor-optical quantum repeater mechanism, apparatus/system employing such a hybrid superconductor-optical quantum repeater mechanism, and method for implementing and using such a hybrid superconductor-optical quantum repeater mechanism.
A quantum computer is a computing device that performs calculations based on the behavior of particles at the sub-atomic level. With quantum computers, the data units, i.e. the quantum bits or “qubits,” can exist in more than one state at a time allowing the quantum computer to have multiple “paths of thought” existing independently of each other even though they all are associated with the same set of particles. In this way, a quantum computer can potentially achieve millions of instructions per second (MIPS) more than known non-quantum computing systems.
The qubit is a binary digit, or bit, similar to that of a classical computer, but that can have several values simultaneously. Essentially, a qubit can be thought of as a particle that has multiple dimensions, each of which can have a high or low state, e.g., a logic 1 or logic 0 state. Hence, two qubits can have four simultaneous and independent states (00, 01, 10, and 11).
In a distributed quantum computing environment, the ability to transfer qubits of data between remote locations is an important factor. With such a distributed system, photons can be transmitted through light conducting fibers or other media to convey quantum information from one quantum computer to another. However, such transmissions are generally subject to signal loss and losses in coherence. For example, as described in Munro et al, “Quantum Repeater,” WO 2007021945 A2, filed Aug. 11, 2006, an optical signal when transmitted 10 km along a typical optical fiber experiences a 1.9 dB loss such that a single photon may have a 50% probability of being lost. Loss and decoherence effects on duplicated or redundant quantum states can create noisy entangled states that may reliably convey quantum information over limited distances, but transmissions over large distances generally require quantum repeaters.
A quantum repeater is a well understood device (see Duan et al., “Long-Distance Quantum Communication with Atomic Ensembles and Linear Optics,” Nature 414, 413-418 (2001)) which permits qubits (understood to be in the form of photons), which are transmitted along a noisy channel, to be largely restored to their original transmitted quantum state by error correction. The repeater is typically envisioned to enable this by bringing the photons to a halt by transforming them to another matter-based form (see Pellizzari et al., “Decoherence Continuous Observation, and Quantum Computing: A Cavity QED Model,” Phys. Rev. Lett. 75, 3788 (1995)) constituting a quantum memory (in present experimental proposals, involving a trapped ion (see Moehring et al., “Entanglement of Single-Atom Quantum Bits at a Distance,” Nature 449, 68-71 (Sep. 6, 2007)) or a semiconductor quantum dot (van Loock et al., “Hybrid Quantum Repeater Using Bright Coherent Light,” Phys. Rev. Lett. 96, 240501 (2006)).
While in memory, these qubits are subject to some quantum logic operations that serve to perform quantum parity checks of various known kinds, which permit the occurrence of errors to be detected and corrected. After these corrective steps, the restored quantum states are again subject to a conversion of embodiment, and the quantum information proceeds on another channel. Either the incoming or outgoing channel may be a teleportation channel, meaning that the photons travel in the direction opposite to that of the quantum information, which is conveyed by the application of a Bell-type quantum measurement, followed by the transmission of classical information in the forward direction.
A quantum repeater is a key element in quantum information processing systems. Its envisioned applications are (1) it can be used to boost the distance and key generation rates at which secure quantum cryptography can operate; (2) it can provide reliable long distance quantum communication for other cryptographic tasks, including secret sharing, quantum data hiding, quantum unlocking, and quantum digital signatures; (3) it can enable other long-distance communication tasks that can be done efficiently with quantum transmissions, including remote memory allocation (sampling complexity), and remote appointment scheduling; and (4) the quantum repeater can be used the distribute any form of quantum computation, so that, for example, a prime factorization problem requiring 109 entangled qubits could be accomplished by interconnecting a network of small processors, each containing, say 105 qubits.
Most previous studies aimed at making a quantum repeater have focused on systems with stationary qubits that are also manipulated optically. The initial theoretical work (described in Pellizzari et al.) envisioned transmitted photons being brought into an optical cavity and caused to interact with a trapped ion or atom. Subsequent work has enlarged this to include trapped atomic clouds (see Sangouard et al. “Robust and Efficient Quantum Repeaters with Atomic Ensembles and Linear Optics,” available at http://arxiv.org/PS_cache/arxiv/pdf/0802/0802.1475v1.pdf), as well as optically addressed quantum dots (see van Loock et al.). No use of superconducting qubits is contemplated in this work. In the area of superconducting qubits, interconversion to light quanta in the GHz frequency range is contemplated (see Majer et al. “Coupling Superconducting Qubits via a Cavity Bus,” Nature 449, 443-447 (Sep. 27, 2007)), however none of these works have contemplated superconducting qubits with interconversion to infrared or visible frequencies.